1. Field of the Invention
This invention relates to a method for determining a desired image signal range representing only a desired part of a radiation image outside of an image part, such as an image part corresponding to a region injected with contrast media, which image part is different from an object image which it is necessary to view, from an image signal detected from a recording medium having the radiation image recorded thereon. This invention also relates to a method for determining a desired image region on a recording medium having a radiation image recorded thereon, which image region carries only a desired image outside of an image part, such as an image part corresponding to a region injected with contrast media, which image part is different from an object image which it is necessary to view.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Pat. Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing systems, in order to eliminate various problems caused by variations in the input information and/or to obtain a radiation image having a good image quality which can serve as an effective tool in, for example, the efficient and accurate diagnosis of an illness, it is desirable to ascertain the characteristics of the image input information before the radiation image is reproduced as a visible image so that the read-out gain can be adjusted to an appropriate value. The characteristics of the image input information depend on the conditions under which an image is recorded, such as the level of the radiation dose used when the image was recorded, on the image input pattern which is determined by what portion of an object (e.g. the chest or the abdomen of a human body) is recorded, and on the image recording method used, such as plain image recording or contrasted image recording. Determining the characteristics of the image input information will hereinafter simply be referred to as "ascertaining the image input information." It is also desirable to adjust the scale factor in accordance with the contrast in the image input pattern in order to optimize the resolution of the reproduced visible image.
The method as disclosed in U.S. Pat. No. 4,527,060 may be used to ascertain the image input information before the visible image is reproduced. In the disclosed method, a preliminary read-out operation (hereinafter referred to as "preliminary readout") is carried out to ascertain the image input information of a radiation image stored on a stimulable phosphor sheet. During the preliminary readout, stimulating rays are used which have an energy level lower than the energy level of the stimulating rays used in a final read-out operation (hereinafter referred to as "final readout"), which obtains a visible image which can be viewed and used, particularly for diagnostic purposes. After the preliminary readout is completed, the final readout is carried out. In the final readout, the read-out gain and/or the scale factor is adjusted to an appropriate value, and/or appropriate signal processing of the image signal is conducted, on the basis of the image input information obtained during the preliminary readout.
Various methods may be used for approximately ascertaining the image input information of the image stored on the stimulable phosphor sheet from a preliminary read-out image signal obtained during the aforesaid preliminary readout. One method is to utilize a histogram (or a probability density function) of the preliminary read-out image signal. Specifically, the image input information can be ascertained from, for example, a characteristic value such as the maximum signal value, the minimum signal value, or a signal value which occurs most frequently, i.e. the signal value corresponding to the maximum value of the histogram. Therefore, when the read-out conditions such as the read-out gain and the scale factor and/or the image processing conditions are adjusted on the basis of the histogram, it is possible to reproduce a visible radiation image having an improved image quality, which makes the image a particularly effective tool in, for example, the accurate and efficient diagnosis of an illness.
On the other hand, in the course of radiation image recording, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Therefore, in many cases, when a radiation image is recorded, a radiation blocking plate made of lead or the like is placed at part of the object. Also, an organ of a human body is often injected with contrast media, such as barium, which have a high radiation absorptivity so that an image of an object portion which it is necessary to view is recorded clearly. The contrast media (specifically, negative contrast media) and the radiation blocking plate are different in their effects, but have high radiation absorptivities. Therefore, the image parts corresponding to the contrast media and the radiation blocking plate have markedly low densities in the reproduced radiation images.
Therefore, in cases where an image of contrast media or a radiation blocking plate has been recorded together with a radiation image of an object on a stimulable phosphor sheet, the image input information of the radiation image stored on the stimulable phosphor sheet is ascertained incorrectly. (A material having a high radiation absorptivity, such as contrast media or a radiation blocking plate is hereinafter referred to as a radiation blocking material.) Specifically, in the aforesaid cases, since components of the image signal representing the image part corresponding to the radiation blocking material are also included in a histogram of the image signal, the signal components having small values (low image density) occur comparatively frequently. Therefore, the image input information is ascertained as if the object image which it is necessary to view had a low image density as a whole.
In order to eliminate the aforesaid problems, a method has heretofore been used wherein a range of image signal components on a low image density side which components represent an image part corresponding to a radiation blocking material is eliminated from a histogram of a preliminary read-out image signal and image input information is ascertained from the remainder of the histogram. However, the range over which the image signal components representing the image part corresponding to a radiation blocking material are distributed varies in accordance with, for example, what portion of an object is recorded and what image recording method is used. Therefore, with the conventional method, it is not always possible to ascertain accurately the range over which the image signal components representing the image part corresponding to a radiation blocking material are distributed (stated reversely, the range over which the image signal components representing a desired image part, which it is necessary to view and which is outside of the image part corresponding to the radiation blocking material, are distributed).